A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.

HVAC and Solar energy engineering Terminology

There are currently 118 terms in this directory beginning with the letter V.
state in which the fluid, usually gas, pressure is lower than atmospheric pressure.

vacuum breaker (vacuum cracking)
a device that allows an increase in the pressure in an enclosure under vacuum by admitting gas (air).

vacuum cooling (vacuum chilling)
cooling by vaporization under vacuum of part of the water contained in the material to be cooled.

vacuum diffusion pump
pump in which the motion of a gas is induced by its diffusion into the vapor phase of a working fluid in the pump.

vacuum evaporation
The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

vacuum freezing
freezing of a substance by lowering pressure to induce vaporization of a part of the solvent (usually water).

vacuum gage
gage for measuring pressures below atmospheric pressure.

vacuum pump
pump for exhausting a system; pump designed to produce a vacuum in a closed system or vessel.

vacuum pump (ion pump)
vacuum pump operated via the ionization of gases, the produced ions being transported by an electrical or magnetic field.

vacuum test
test to check the gas tightness of an uncharged system or component by drawing a vacuum on it.

vacuum zero
The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

valence band
The highest energy band in a semiconductor that can be filled with electrons.

valence level energy/valence state
Energy content of an electron in orbit about an atomic nucleus. Also called bound state.

device to regulate or stop the flow of fluid in a pipe or a duct by throttling.

valve area
area encountered by a fluid flowing through a valve.

valve body
shell or housing of a valve into which the mechanism for regulating flow is fitted.

valve cage
device for holding and locating a valve disc, poppet, etc., in relation to a valve seat.

valve cover
removable part that allows access to the valve mechanism.

valve disc
disc or button fixed to the end of a valve stem and which meets the valve seat on closure.

valve flutter (valve bounce)
uncontrolled oscillation of a compressor valve on its seat during opening or closing.

valve guard
part that limits or controls the opening of a valve.

valve lift
vertical distance through which a valve is raised to allow the passage of a fluid.

valve obturator
that part of a valve that closes off or stops fluid flow.

valve plate
plate on which the valves of a reciprocating or rotary compressor are located.

valve plug
(1) a movable part that provides a variable restriction in a port. (2) generic name for that part of a valve which mates with the seat to control or shut off flow.

valve port
opening for passage of a fluid in a valve.

valve seat
stationary portion of the valve which, when in contact with the movable portion, stops flow completely.

valve stem
spindle movable towards or away from a valve seat for guiding the valve head during closing or opening.

valve trim
internal parts (stem, plug, disc, seat, etc.) of a valve.

Van’t Hoff’s principle
if the temperature of interacting substances in equilibrium is raised, the equilibrium concentrations of the reaction are changed, so that the products of that reaction that absorb heat are increased in quantity, or if the temperature for such an equilibrium is lowered, the products that evolve heat in their formation are increased in amounts.

vane ratio
(1) in air-distributing devices, the ratio of the depth of vane to shortest opening width between two adjacent bars in a grille. (2) ratio of the chord length to the vane pitch.

vaneaxial fan
fan that moves air in the general direction of the axis about which it rotates, parallel to the shaft, and flow through the centrifugal fan wheel, impeller, or propeller is substantially radial. The fan is typically mounted in a round duct or tube to improve pressure characteristics and has vane-type straighteners to remove much of the swirl or spin of the air created by the fan wheel to improve fan efficiency. Fans can be either direct drive or belt driven.

(1) gas, particularly one near equilibrium with its liquid phase and one that does not follow the gas laws. The term is usually used instead of gas to refer to a refrigerant, or in general, to any gas below the critical temperature. (2) substance in the gaseous state that can also exist as a liquid or solid at normal atmospheric conditions. Compare to [[gas]].

vapor barrier
use vapor retarder. See [[water vapor retarder]].

vapor composition
the concentrations (mass percentages) of components present in the vapor phase of a refrigerant blend that is in equilibrium with its liquid phase.

vapor concentration
(also known as absolute humidity or water vapor density number), in a mixture of water vapor and dry air, the mass of water vapor in a specific volume of the mixture. Compare to [[relative humidity]].

vapor content ratio
ratio of the amount of vapor present compared to the amount present at saturated conditions.

vapor control
(1) control device with a vapor charged power element. (2) method of control using the expansion and contraction forces of a contained vapor.

vapor jet
refrigerating cycle using an ejector to compress the refrigerant vapor from the evaporator to the condenser. The term steam jet is used when water is the refrigerant.

vapor lock
formation of vapor in a liquid line that reduces mass flow compared to the flow of the total liquid at the same pressure differential.

vapor lock device
orifice capillary tube, or other device having a restricted passage of fixed size for liquid refrigerant. It restricts flow of vapor of that same liquid to a lower rate of flow with the same pressure difference.

vapor mass fraction
see [[vapor quality]].

vapor pickup tube
usually located at the bottom of large absorber tube bundles.

vapor pressure
the pressure exerted by a vapor. If a vapor is kept in confinement over its liquid so that the vapor can accumulate above the liquid with the temperature constant, the vapor pressure reaches a maximum called the saturated vapor pressure.

vapor quality
ratio of the mass of vapor to the total mass when a substance exists as part liquid and part vapor at the saturation temperature. Note: quality has meaning only when the substance is in a saturated state, i.e., at saturation pressure and temperature.

vapor refrigerating system
system using a condensable vapor as the refrigerant.

vapor retarder
material or construction that adequately impedes the transmission of water vapor under specified conditions. Compare to [[structural barrier]].

vapor sample
a sample taken from the vapor phase of a refrigerant blend.

vapor-pressure thermometer
one based on pressure built up by the vapor evolved by liquid in a sealed container.

change of phase from liquid to vapor.

vaporizing burner
burner designed to utilize the temperature of the heated combustion chamber to vaporize the liquid fuel fed into such chamber.

variable flow
throttling control of water during a cooling or heating process.

variable output
for each element of a control system, a physical quantity whose changes are governed ultimately by the functioning of the element.

variable refrigerant flow (VRF) system
an engineered direct-expansion (DX) multisplit system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to multiple indoor fan-coil units, each capable of individual zone temperature control, through integral zone temperature control devices and common communications network. Variable refrigerant flow utilizes three or more steps of control on common, interconnecting piping.

variable volume, variable temperature (VVT)
combination of varying both airflow and temperature in response to space load, for the purpose of resetting temperature to maintain greater low-load airflow to the space than in a true variable volume system.

variable-capacity equipment
heating and cooling equipment that operates in stages of different capacity depending on building load, e.g., electric furnaces with several separate heater elements.

variable-frequency drive (VFD)
electronic device that varies its output frequency to vary the rotating speed of a motor, given a fixed input frequency. Used with fans or pumps to vary the flow in the system as a function of a maintained pressure.

variable-speed control of a fan
a method of controlling airflow produced by a fan by means of adjusting the speed or revolutions per minute (rpm) of the fan.

variable-speed drive
see [[variable-frequency drive]].

A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

VAV box
variable-air volume terminal device

a measurement of the distance traveled per unit of time. This quantity is defined by its magnitude and direction at any point of the flow.

velocity coefficient
ratio of the actual velocity of gas emerging from a nozzle to the velocity calculated under ideal conditions; it is less than one because of friction losses.

velocity constant
ratio of the rate of change of the input command signal to the steady-state error, in a control system where these two quantities are proportional.

velocity head
see [[velocity pressure]].

velocity pressure
(1) in a moving fluid, the pressure that would induce an equivalent velocity if applied to move the same fluid through an orifice, so that all pressure energy is converted into kinetic energy. Velocity pressure is always a positive value, in. H2O (kPa). (2) the difference between the total pressure and static pressure (relative to the same datum).

velocity profile
graph that represents, in a plane section, the velocity distribution in a flowing fluid.

velocity reduction method duct sizing
method in which ducts are sized so that selected velocities occur in specific duct lengths.

vena contracta
smallest area of a fluid stream leaving an orifice.

a connection in any system or enclosure that may be open to atmosphere or to a lower pressure, space, or vessel and is intended for the transfer of any fluid. A vent can be used for either intake or relief purposes.

vent connector
the portion of the venting system that connects the gas appliance or its draft hood to the chimney or vent terminal.

vent damper
a device intended for installation in the venting system of an individual, automatically operated, fossil-fuel-fired appliance in the outlet or downstream of the appliance draft control device, which is designed to automatically open the venting system when the appliance is in operation and to automatically close off the venting system when the appliance is in a standby or shutdown condition.

vent gas
see [[flue gas]].

vent limiter
a device that limits the flow of air from the atmospheric diaphragm chamber of a gas pressure regulator to the atmosphere. A vent limiter may be a limiting orifice or other limiting device.

vent or air intake terminal
a device that is located on the outside of a building and may be connected to a furnace or boiler by a system of conduits. It is composed of an air intake terminal through which the air for combustion is taken from the outside atmosphere and an exhaust terminal from which flue gases are discharged.

vent pipe
(1) discharge piping connected to a safety or relief valve. (2) passages and conduits in a direct vent or direct exhaust system through which gases pass from the combustion chamber to the outdoor air.

vent relief
(1) opening in a tank, duct, or other piece of equipment, sealed to prevent escape of material within normal pressures, but arranged to open automatically to relieve excessive pressure. (2) relief opening in a pressure regulator, normally open to the atmosphere.

vented cell
A battery designed with a vent mechanism to expel gases generated during charging.

vented crawlspace
a crawlspace with an open vent area 1/150th of its floor area with vents distributed over all exterior surfaces.

(1) the process of supplying air to or removing air from a space for the purpose of controlling air contaminant levels, humidity, or temperature within the space. (2) the process of supplying or removing air by natural or mechanical means to or from any space. Such air is not required to have been conditioned.

ventilation air
the minimum amount of outdoor air required for the purpose of controlling air contaminant levels in buildings.

ventilation effectiveness
(1) (Ev), evaluation of air delivery, regardless of the effectiveness of contaminant removal. (2) (Ev), fraction of the outdoor air that reaches the occupied zone of the destined space.

ventilation effectiveness concentration
measure of the relationship between the pollutant concentration in the exhaust air and the pollutant concentration in the specified zone.

contraction in a pipeline or duct that increases the fluid velocity to lower its static pressure, followed by a gradual expansion to allow recovery of static pressure. Used for metering and other purposes that involve change in pressure.

Venturi tube
(1) specially shaped construction inserted in a pipe to measure the rate of flow from induced changes in pressure in the constricted section. (2) specially shaped construction that creates a vacuum in a steam/water refrigeration system.

the process by which specific documents, components, equipment, assemblies, systems, and interfaces among systems are confirmed to comply with the criteria described in the rating program.

vertical axis of measurement (y-axis)
an axis of measurement perpendicular to the horizontal and axial axes of measurement of a piece of equipment but not necessarily in the horizontal plane.

vertical multijunction (VMJ) cell
A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See also multijunction device and split-spectrum cell.

very cold climate
climates that have more than 9000 annual heating degree-days base 65°F-day (5000 annual heating degree-days base 18°C-day).

container or structural envelope in which materials are processed, treated, or stored. Compare to [[tank]].

an oscillation wherein the quantity is a parameter that defines the motion of a mechanical (physical) system. The term is usually used to mean an objectionable movement.

vibration isolation
in any rotating equipment where the isolation of its vibration is by devices external to the unit. Note: examples: flex duct connections, pipe flex connections, and spring isolators.

vibration severity
the magnitude of the vibration expressed in engineering units (usually velocity [mm/s]). Note: vibration severity values may represent either the overall value or a value obtained from a frequency analysis. It is extremely important to denote which choice was made. It is also necessary to denote whether the value is zero-to-peak, peak-to-peak, or root mean square (rms).

instrument for measuring viscosity.

(1) property of a fluid to resist flow or change of shape. (2) property of semifluids, fluids, and gases by which they resist an instantaneous change of shape or arrangements of parts. It causes fluid friction whenever adjacent layers of fluid move with relation to each other.

viscosity coefficient
force per unit area required to produce unit relative viscosity between two parallel areas of fluid unit distance apart.

viscosity index (VI)
(1) characteristic of lubricating oil that indicates the change in viscosity with temperature; i.e., a high VI oil has less change in viscosity with temperature change than a lower VI oil. (2) empirical number evaluated by comparing the viscosity of a substance with that of a standard substance.

viscosity manometer
pressure gage which uses the variations of viscosity of a gas with pressure (in vacuum techniques).

viscous filter
filter in which the medium is made from materials which have been impregnated with a viscous oil.

viscous flow
(1) laminar flow or streamline flow. (2) type of gas flow in which the average free path of gas molecules is much smaller than the smallest cross-sectional dimension of the pipe conveying the substance.

volatile liquid
liquid that evaporates readily at atmospheric pressure and room temperatures.

volatile organic compounds (VOC)
organic compounds in the vapor state present in an indoor atmosphere.

volatile refrigerant
a refrigerant that changes from the liquid to the vapor state in the process of absorbing heat.

volt (V)
A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

volt ohmmeter (VOM)
a device for measuring voltage, or the resistance of an electrical circuit. Some models also measure current flow through components in electrical circuits.

electric potential or potential difference expressed in volts.

voltage at maximum power (Vmp)
The voltage at which maximum power is available from a photovoltaic module.

voltage drop
a decrease in voltage caused by losses in the lines connecting the power source to the load.

voltage protection
Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

voltage regulation
This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

voltage relay
(1) device that is voltage sensitive with the ability to sense between two or more voltage levels to operate a set of contacts. (2) relay that responds to a predetermined voltage.

voltampere reactive (VAR)
unit of voltampere reactive power. For a two wire circuit, the product of the voltage times the current times the sine of the angular phase difference by which the voltage leads or lags the current. VARs and watts combine in a quadrature to form voltamperes.

volume control damper
device mounted in a duct or opening used to vary the volume of air flowing through.

volumetric air flow rate
volumetric flow of air over specified time, usually expressed in l/s or m3/h.

volumetric efficiency due to cylinder heating
ratio of the total to the apparent volumetric efficiency. Also called real or no-clearance volumetric efficiency.

volumetric heat capacity
the change in heat stored in unit volume of material for unit change of temperature.

volumetric moisture capacity
the change in stored moisture per unit volume of porous material and per unit moisture potential change.

volumetric total efficiency
ratio of the actual volume of gas moved by the compressor or pump to actual displacement of the compressor or pump.